Radioactivity

nucleon number

mass number

isotopes of hydrogen

protium, deuterium, tritium

Effect of energy shell on electron

electrons in higher energy shells have greater potential energy as they have more distance between them and the nucleus

What happens when electrons absorb EM radiation

they gain energy and move to a higher energy level further from the nucleus

What happens when electrons emit EM radiation

they lose energy and move to a lower energy level closer to the nucleus

How can radiation ionise atoms?

It can knock 1 or more electrons out of the orbit, leaving the atom positively charged

Rutherford's Gold Foil Experiment

Alpha-particles were directed at a thin sheet of gold foil; most passed through the foil(shows atoms are mostly empty space), but a few were deflected(shows there must be an area of concentrated positive charge in the atom - the nucleus);

JJ Thomson model

Plum pudding mode - negatively charged electrons scattered in positive matter

Bohr model

model of the atom in which electrons move rapidly around the nucleus in paths called orbits

Why are some isotopes unstable?

Large size or number of protons and neutrons are out of balance

What happens to an unstable nucleus?

Decays, emitting nuclear radiation

Different types of nuclear radiation

Alpha particles, beta minus and beta plus particles, gamma rays, neutron emission

alpha particle

A cluster of 2 protons and 2 neutrons (a helium nucleus) with a charge of plus 2

What can alpha and beta particles be affected by?

An electric field

Beta particles (plus and minus)

Beta particles are fast-moving electrons emitted from the nucleus

Beta-plus particles are fast-moving positrons (positive version of electrons)

They are produced in nuclei when a neutron changes into a proton and an electron

Beta-minus particles have a charge of -1

Beta-plus particles have a charge of +1

This means they can be affected by an electric field

gamma rays

The symbol for gamma is γ

Gamma rays are electromagnetic waves

They have the highest energy of the different types of electromagnetic waves

Gamma rays have no charge

neutron emission

The symbol for a neutron is n

Neutrons are one of the two particles found in the nucleus of atoms

Neutrons are neutral, they have no charge

Most ionising form of radiation and why?

Alpha radiation is the most ionising form of nuclear radiation

This is because alpha particles have a charge of +2 and is the heaviest

Least ionising form of radiation

Gamma

Radiation from most to least penetrating power

gamma, beta, alpha

range of gamma in air

infinite

range of alpha in air

few cm

range in air of beta

10s of cm

What material is alpha stopped by?

Alpha is stopped by paper

What material is beta stopped by?

Beta is stopped by a few millimetres of aluminium

What material is gamma stopped by?

Gamma can be reduced by several metres of concrete or several centimetres of lead

Examples of natural sources of radiation

Rocks - radon gas can be emitted as a result of rock decay i.e. uranium

Cosmic rays from space - can lead to gamma ray production as a result of air molecule collision

Foods - bananas contain potassium 40 - small radiation amount

How to determine corrected count rate

You must account for background radiation

Subtract readings with no radioactive source present from readings with the source present

man made nuclear sources

x rays

nuclear waste

nuclear weapons

How does photographic film detect radiation?

becomes darker when it absorbs radiation, just like it does when it absorbs visible light

The more radiation the film absorbs, the darker it is when it is developed

Where is photographic film used?

People who work with radiation i.e. radiographers, wear photographic film badges which are checked regularly to monitor the levels of radiation absorbed

What is radiation received by a person measured in?

Sieverts(Sv)

How can radiation be measured?

Geiger-Muller tube and counter

How do GM tubes work?

Each time it absorbs radiation, it transmits an electrical pulse to a counting machine

This makes a clicking sound or displays the count rate

The greater the frequency of clicks, or the higher the count rate, the more radiation the Geiger-Müller tube is absorbing.

What kinds of nuclei emit beta minus particles?

nuclei that have too many neutrons

Nucleus changes during beta minus decay

a neutron turns into a proton and an electron

example of beta decay equation - starting with carbon 14 isotope

¹⁴₆C → ¹⁴₇N + e⁻

What kinds of nuclei emit beta plus particles?

nuclei that have too many protons

Nucleus changes during beta plus decay

A proton turns into a neutron and a positron

Beta plus decay equation

p -> n + e+

Changes in mass and atomic no for each type of radiation

alpha - mass -4 atomic -2

beta- - mass 0 atomic +1

beta- - mass 0 atomic -1

gamma - mass 0 atomic 0

neutron - mass -1 atomic0

What can happen to nuclei when undergoing decay involving nucleus rearrangement

They can emit gamma radiation as well

How to reporesent alpha, beta and gamma decay in an equation

the product of the equation should have an alpha particle, or a beta minus or plus with -1 or +1 charge or gamma with neither charge nor man

half life

The time taken for half the undecayed nuclei to decay or the activity of a source to decay by half

What does a short half life suggest?

the nuclei will decay very quickly, it will emit a lot of radiation in a short amount of time - a small amount won't be as bad as the material will quickly lose its radioactivity, if a large amount is used, handling it becomes very dangerous

What does a long half life suggest?

the nuclei will decay slowly, it won't emit a lot of radiation, but will be radioactive for a very long time - it poses a risk of contamination for a very long time

Nature of radioactive decay

- Completely random

- spontaneous as you cannot force a nuclei to decay more than another

- For a particular nuclei there is a probability that it will decay

nuclear activity

The rate at which the unstable nuclei from a source of radiation decays

What is nuclear activity measured in?

Becquerels (Bq)

What is 1 becquerel equal to?

1 nucleus in the source decaying in 1 second

How to determine half life from an activity time graph

record the time for 1 activity level and record the time for half of that activity level, the half life is the difference in hrs

Dangers of ionising radiation

Tissue damage and mutations of DNA in cells causing tumours that can lead to cancer

How to handle radiation safely

- Radioactive sources should be kept in a shielded container when not in use, for example, a lead-lined box

- Radioactive materials should only be handled when wearing gloves, and with tongs to increase the distance from them

- It may be appropriate to wear protective clothing to prevent the body becoming contaminated

- The time that a radioactive source is being used for should be limited

Factors of the amount of radiation that a person receives

occupation, lifestyle or location

contamination

The unwanted presence of materials containing radioactive atoms on other materials

When does contamination occur?

when a radioactive isotope gets onto a material where it should not be, often due to a radiation leak

irradiation

The process of exposing a material to alpha, beta or gamma radiation

differences between radioactive nature of irradiation and contamination

- irradiation doesn't make the material radioactive, unlike contamination, but irradiaton can kill living cells

differences between prevention of irradiation and contamination

Irradiation: prevented by using shielding such as lead clothing which absorbs most of the radiation that would hit a person

Contamination: prevented by safely handling sources and an airtight suit which prevents radioactive atoms from getting inside the person

Uses of radiation(5)

- Medical procedures including diagnosis and treatment of cancer

- Sterilising food (irradiating food)

- Sterilising medical equipment

- Checking the thickness of materials

- Smoke detectors (alarms)

How is radiation used in smoke detectors?

Alpha particles are used in smoke detectors

The alpha radiation will normally ionise the air within the detector, creating a current

The alpha emitter is blocked when smoke enters the detector

The alarm is triggered by a microchip when the sensor no longer detects alpha

How is radiation used in measuring material thickness?

As a material moves above a beta source, the particles that are able to penetrate it can be monitored using a detector

If the material gets thicker, more particles will be absorbed, meaning that less will get through

If the material gets thinner the opposite happens

This allows the machine to make adjustments to keep the thickness of the material constant

Why is beta particles specifically used in measuring material thickness?

Beta radiation is used because it will be partially absorbed by the material

If alpha particles were used all of them would be absorbed and none would get through

If gamma were used almost all of it would get through and the detector would not be able to sense any difference if the thickness were to change

How is radiation used in diagnosing and treating cancer?

Radiotherapy is the name given to the treatment of cancer using radiation

(Chemotherapy is treatment using chemicals)

Although radiation can cause cancer, it is also highly effective at treating it.

Radiation can kill living cells. Some cells, such as bacteria and cancer cells, are more susceptible to radiation than others

Beams of gamma rays are directed at the cancerous tumour

Gamma rays are used because they are able to penetrate the body, reaching the tumour

The beams are moved around to minimise harm to healthy tissue whilst still being aimed at the tumour

A tracer is a radioactive isotope that can be used to track the movement of substances, like blood, around the body

A PET scan can detect the emissions from a tracer to diagnose cancer and determine the location of a tumour.

What is the preferred half life of isotopes in tracers used in PET scans?

short - long enough to carry out the procedure but not too long that they cause long term harm

Where must tracers be produced relatuve to the hospital?

Nearby as they have a short half life

How do PET scans detect tumours?

The tracer decays into a positron that collides into a electron that releases gamma waves that can be detected in the body

How is radiation used in sterilising food and medical equipment?

Gamma radiation is widely used to sterilise medical equipment

Gamma is most suited to this because:

It is the most penetrating out of all the types of radiation

It is penetrating enough to irradiate all sides of the instruments

Instruments can be sterilised without removing the packaging

Food can be irradiated in order to kill any microorganisms that are present on it

This makes the food last longer, and reduces the risk of food-borne infections

nuclear fission

One large, unstable, parent nucleus splits into two smaller daughter nuclei

How does nuclear fission work in reactors?

A neutron is fired at the parent nucleus, often uranium 235 or plutonium.

This causes said nucleus to be very unstable and to split by nuclear fission almost immediately into 2 smaller daughter nuclei and 2 or 3 neutrons which can start another fission reaction wiith other uranium/plutonium nuclei

This leads to a chain reaction, the fission products move away very quickly and energy is transferred from the nuclear store to the kinetic store of the products and this energy can be used to generate electricity

purpose of control rods in reactors

Absorbing neutrons:

Control rods are made of a material which absorb neutrons without becoming dangerously unstable themselves

The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods

Lowering the rods further decreases the rate of fission, as more neutrons are absorbed

Raising the rods increases the rate of fission, as fewer neutrons are absorbed

This is adjusted automatically so that exactly one fission neutron produced by each fission event goes on to cause another fission

In the event the nuclear reactor needs to shut down, the control rods can be lowered all the way so no reaction can take place

purpose of moderators in reactors

To slow down neutrons

The moderator is a material that surrounds the fuel rods and control rods inside the reactor core

The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum

The neutrons are slowed down so that they are in thermal equilibrium with the moderator, hence the term 'thermal neutron'

This ensures neutrons can react efficiently with the uranium fuel

How is eletricity generated from nuclear power stations?

Nuclear fission produces a lot of heat which is carried away from the reactor by a coolant

The coolant then heats a separate water source turning it into steam

The steam is then used to drive turbines which then turn generators and produce electricity

Advantages of nuclear power stations(3)

- they do not emit any CO₂ or other gases

- far less fuel is required as uranium provides much more energy per kg compared to coal and fossil fuels

- highly reliable for electricity production

Disadvantages of nuclear power stations(4)

- Waste is radioactive and safe disposal is very difficult and expensive.

- Local thermal pollution from wastewater affects marine life.

- Large-scale accidents can be catastrophic.

- Public perception of nuclear power is negative.

- Costs of building and safely shutting down are very high.

fusion

When two light nuclei join to form a heavier nucleus

Why is nuclear fusion hard to achieve on earth and isn't used as a current energy source?

Requires extremely high temperatures and pressure

Whilst attaining these factors is possible, containing them is difficult

Furthermore, the energy required to achieve said conditions outweighs the energy generated from fusion so overall it isn't bringing benefit and is thus unused.

How do stars such as the sun use fusion?

2 hydrogen atoms fuse to make helium and produces lots of energy - this is important to life on earth.

Fusion vs Fission

joining atoms vs splitting atoms.

both release energy, though fusion provides more energy/kg

fusion requires v high temps, but fission requires a high temp and a neutron to start the chain reaction

conditions for fusion

Very high temperature of fuel

Very high kinetic energy / speed of nuclei to overcome repulsion

Very high density / pressure to increase the possibility of suitable collisions